While building a nuclear weapon is a complex task in and of itself, the most difficult part is refining uranium. In this informative video, Bill Hammack explains.
Weapons and power plants require uranium that contains a greater amount of the isotope uranium-235 than found in natural uranium, which is mostly uranium-238. He outlines the key difficulty in separating the two isotope: They have nearly identical properties. He explains the two key methods for separation: Gas diffusion and centrifuges.